Phosphors have been utilized for vacuum fluorescent displays (VFD), field emission displays (FED), plasma display panels (PDP), cathode ray tubes (CRT), white light emitting diodes (LED), and the like.
In all these usages, it is required to supply an energy to an applicable phosphor to thereby excite it so as to cause it to emit light, and the phosphor is excited by an excitation source having a higher energy such as vacuum ultraviolet light, ultraviolet light, electron beam, blue light, or the like, such that the phosphor is caused to emit visible light.
However, since phosphor is exposed to the aforementioned excitation sources to resultingly cause a problem of deteriorated luminance, thereby necessitating a phosphor, which is free of luminance deterioration. As such, there has been proposed a sialon phosphor as a phosphor exhibiting less luminance deterioration, instead of the conventional silicate phosphor, phosphate phosphor, aluminate phosphor, sulfide phosphor, and the like.
The sialon phosphor is produced by a production process as generally described below.
Firstly, there are mutually mixed silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3), and europium oxide (Eu2O3) at a predetermined molar ratio, followed by holding for 1 hour at a temperature of 1,700° C. in nitrogen at 1 atm (0.1 MPa), and firing by hot pressing for production (see patent-related reference 1, for example).
It has been reported that α-sialon obtained by the process and activated by Eu ion is established into a phosphor, which is excited by blue light at 450 to 500 nm and caused to emit yellow light at 550 to 600 nm.
However, there have been demanded not only the phosphor which emits yellow light but also phosphor which emit orange light and red light, respectively, for usages such as white LED and plasma display each having an ultraviolet LED as an excitation source. Further, there have been demanded phosphor which emit orange light, red light, and the like, respectively, in a white LED having a blue LED as an excitation source, for an improved color rendering property.
As a phosphor, which emits red light, there has been reported an inorganic substance (Ba2-xEuxSi5N8; where x=0.14 to 1.16) obtained by activating a Ba2Si5N8 crystal with Eu, in a scientific literature (see patent-unrelated reference 1) prior to filing of the present application.
There has been further reported a phosphor including, as a host material, a ternary nitride MxSiyNz (M=Ca, Sr, Ba, Zn; where x, y, and z take various values, respectively) of alkali metals and silicon at various compositions, in the second chapter of a publication “On new rare-earth doped M-Si—Al—O—N materials” (see patent-unrelated reference 2).
Similarly, there has been reported MxSiyNz:Eu (M=Ca, Sr, Ba, Zn; where z=2/3x+4/3y), in U.S. Pat. No. 6,682,663 (patent-related reference 2).
As sialon phosphor, nitride phosphor, and oxynitride phosphors different from the above, there have been described phosphors including, as host crystals, MSi3N5, M2Si4N7, M4Si6N11, MgSi11N23, M16Si15O6N32, M13Si18Al12O18N36, MSi5Al2ON9, and M3Si5AlON10 (where M is Ba, Ca, Sr, or rare earth element) activated with Eu, Ce, or the like in JP-A-2003-206481 (patent-related reference 3), and there has been also described therein a phosphor which emits red light.
Further, LED lighting units utilizing these phosphors are known.
Furthermore, there has been reported a phosphor obtained by activating an Sr2Si5N8 or SrSi7N10 crystal with Ce, in JP-A-2002-322474 (patent-related reference 4).
In JP-A-2003-321675 (patent-related reference 5), there have been found a description of a phosphor represented by LxMyN(2/3x+4/3y):Z (L is a divalent element such as Ca, Sr, Ba, or the like, M is a tetravalent element such as Si, Ge, or the like, and Z is an activator such as Eu), and a description that addition of a small amount of Al brings about an effect of restricting afterglow.
Further, it has been known that a combination of the phosphor with a blue LED provides a light emitting apparatus for emitting warm color based and slightly reddish white light.
In turn, there has been reported a phosphor configured with various L elements, M elements, and Z elements, as an LxMyN(2/3x+4/3y):Z phosphor, in JP-A-2003-277746 (patent-related reference 6).
Meanwhile, although JP-A-2004-10786 (patent-related reference 7) has described a wide variety of combinations concerning L-M-N:Eu, Z types, it has failed to show an effect of improved emission characteristics in case of adopting specific compositions or crystal phases as host materials.
Although the phosphors represented by those of the aforementioned patent-related references 2 through 7 have been reported as ones including various different crystal phases as host materials such that nitrides of divalent elements and tetravalent elements are included as host crystals while providing known phosphors for emitting red light, emission luminances of red light have been insufficient insofar as based on excitation by blue visible light.
Further, the phosphors have been chemically unstable depending on compositions, thereby exhibiting a problem of durability.
Meanwhile, as a lighting apparatus, there has been known a white light emitting diode based on a combination of a blue light emitting diode element with a blue-light absorbing/yellow-light emitting phosphor, which has been practiced in various lighting usages.
Representative examples thereof include JP-2900928 (patent-related reference 8) entitled “Light Emitting Diode”, JP-2927279 (patent-related reference 9) entitled “Light Emitting Diode”, JP-3364229 (patent-related reference 10) entitled “Casting Material for Wavelength Conversion, Production Method Thereof, and Light Emitting Element”, and the like.
The phosphors, which are particularly frequently utilized in these light emitting diodes, are yttrium/aluminum/garnet based phosphors represented by a general formula (Y, Gd)3(Al, Ga)5O12:Ce3+.
However, the white light emitting diode comprising the blue light emitting diode element and the yttrium/aluminum/garnet based phosphor has a feature to emit bluish white light due to lack of a red component, thereby problematically exhibiting deviation in a color rendering property.
Under such circumstances, there has been investigated a white light emitting diode including two kinds of mixed and dispersed phosphors, such that a red component lacking in case of a yttrium/aluminum/garnet based phosphor is compensated for by an additional red-aimed phosphor.
Examples of such light emitting diodes include those described in JP-A-10-163535 (patent-related reference 11) entitled “White Light Emitting Element”, JP-A-2003-321675 (patent-related reference 5) entitled “Nitride Phosphor and Production Method Thereof”, and the like.
However, there have been still left problems to be improved concerning an insufficient color rendering property even by these inventions, thereby necessitating a problem less light emitting diode.
Further, the red-aimed phosphor described in JP-A-10-163535 (the patent-related reference 11) includes cadmium, thereby exhibiting a problem of environmental pollution. Contrary, although the red-light emitting phosphor including Ca1.97Si5N8:Eu0.03 described in JP-A-2003-321675 (the patent-related reference 5) as a representative example do not include cadmium, the phosphors are low in luminance, thereby still necessitating a further improvement of emission intensities thereof.
Referenced Literature/Publication:
Patent-unrelated reference 1: H. A. Hoppe, and four others, “Journal of Physics and Chemistry of Solids”, 2000, Vol. 61, pp. 2001-2006
Patent-unrelated reference 2: “On new rare-earth doped M-Si—Al—O—N materials”, written by J. W. H. van Krevel, TU Eindhoven 2000, ISBN 90-386-2711-4
Patent-related reference 1: JP-A-2002-363554
Patent-related reference 2: U.S. Pat. No. 6,682,663
Patent-related reference 3: JP-A-2003-206481
Patent-related reference 4: JP-A-2002-322474
Patent-related reference 5: JP-A-2003-321675
Patent-related reference 6: JP-A-2003-277746
Patent-related reference 7: JP-A-2004-10786
Patent-related reference 8: JP-2900928
Patent-related reference 9: JP-2927279
Patent-related reference 10: JP-3364229
Patent-related reference 11: JP-A-10-163535